U.S. patent application number 10/470824 was filed with the patent office on 2004-05-20 for electrolyte solution for electrochemical cells.
Invention is credited to Schwake, Andree.
Application Number | 20040096747 10/470824 |
Document ID | / |
Family ID | 7672133 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040096747 |
Kind Code |
A1 |
Schwake, Andree |
May 20, 2004 |
Electrolyte solution for electrochemical cells
Abstract
Electrolyte solutions were suggested for electrochemical cells,
for example for double-layer capacitors, which showed
conductivities of more than 20 mS/cm at 25.degree. C., at least
comprising of a primary salt, which is released in a solvent alloy
of "A" at least a solvent of high polarity and "B" at least a
non-toxic solvent of low viscosity. Because of the low or
non-availability of parts of acetonitrile, the electrolyte
solutions are not in danger of a release of hydrogen cyanide if
fire breaks out.
Inventors: |
Schwake, Andree;
(Heidenheim, DE) |
Correspondence
Address: |
FISH & RICHARDSON PC
225 FRANKLIN ST
BOSTON
MA
02110
US
|
Family ID: |
7672133 |
Appl. No.: |
10/470824 |
Filed: |
December 29, 2003 |
PCT Filed: |
January 23, 2002 |
PCT NO: |
PCT/DE02/00221 |
Current U.S.
Class: |
429/326 ;
252/62.2; 361/502; 361/504; 429/329; 429/330; 429/331; 429/332 |
Current CPC
Class: |
Y02E 60/13 20130101;
H01G 11/60 20130101; H01G 9/038 20130101; H01G 11/62 20130101 |
Class at
Publication: |
429/326 ;
429/329; 429/330; 429/332; 429/331; 361/502; 361/504;
252/062.2 |
International
Class: |
H01M 010/40; H01G
009/035; H01G 009/155 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 30, 2001 |
DE |
101 03 994.8 |
Claims
1. Electrolyte solution for electrochemical cells with a
conductivity of more than 20 mS/cm at 25.degree. C. showing
following components A at least a solvent of high polarity with a
DK>10, B at least a solvent of lower viscosity <1 cP C at
least a primary salt.
2. Electrolyte solution according to one of the above claims, in
which Component A covers at least a solvent of high polarity that
is selected from nitrile, lactone, carbonate, sulfone,
oxazolidinone, imidazolidinone, pyrrolidone or amides in the
Component B at least a solvent of lower viscosity, which is
selected from open-chained carbonates, ketones, aldehydes, ester or
substituted benzenes.
3. Electrolyte solution according to one of the above claims where
Component A is included in a part of at least 30 weight %.
4. Electrolyte solution according to one of the above claims, in
which Component A comprises of at least a cyclical carbonate, which
in the entire electrolyte solution has a portion of at least 40
weight %.
5. Electrolyte solution according to one of the above claims, in
which primary salts are contained as Component C, which exist at
room temperature as a fluid or melted.
6. Electrolyte solution according to one of the above claims, in
which the primary salt comprises in the Component C and is selected
from a combination of the following anions and cations: Anions:
PF.sub.6--, AsF.sub.6--, SO.sub.2CF.sub.3--.
N(SO.sub.2CF.sub.3).sub.2--, C(SO.sub.2CF.sub.3).sub.3--,
BOR.sub.4--, BF.sub.4--, ClO.sub.4--, AlCl.sub.4-- or fluor alkyl
phosphate, where R is an alkyl residue, Cations:
(C.sub.2H.sub.5).sub.4N.sup.+, (C.sub.2H.sub.5).sub.3CH.sub.3N.s-
up.+, Li.sup.+, imidazolium, pyrrolidinium, pyridineium, or
morpholinium.
7. Electrolyte solutions according to one of the above-mentioned
claims, according to which the Component C is Triethyl methyl or
Tetra ethyl ammonium tetra fluoro borate.
8. Electrolyte solution according to one of the above-mentioned
claims, according to which the Component A is selected from one
group, which contains the following solvent: Propylene carbonate,
Ethylene carbonate, 3-Methyl-2-Oxazolidinone, .gamma.-butyrolactone
or acetonitrile, according to which the Component B is selected
from one group, which contains the following solvent: acetone,
methyl formate, ethyl acetate, y-butyrolactone, acetonitrile or
ethyl methyl ketone.
9. Electrolyte solution according to one of the above claims, in
which propylene carbonate and ethylene carbonate is present in
Component A with a share of about 40 weight % each, in which
acetonitrile is present in Component B with a share of approx. 20
weight %.
10. Electrolyte solution according to claims 1 to 8, in which
ethylene carbonate is represented in Component A with a share of
approx. 37 weight % in which Component B is represented by a
compound of acetonitrile with a share of approx. 26 weight % and
diethyl carbonate with a share of approx. 37 weight %.
11. Electrolyte solution according to claims 1 to 8, in which
Component A is represented by a compound of propylene carbonate
with a share of about 24 weight % and ethylene carbonate with a
share of about 25 weight % is represented, in which Component B is
represented by a compound of acetonitrile with a share of approx.
26 weight % and acetone with a share of approx. 25 weight %.
12. Electrolyte solution according to claims 1 to 8, in which
Component A is represented by a compound of propylene carbonate
with about 40 weight % and ethylene carbonate with a share of about
20 weight %, in which Component B is represented by acetonitrile
and y-butyrolactone with a share of approx. 20 weight %.
13. Electrolyte solution according to claims 1 to 8, in which
ethylene carbonate is represented in Component A with a share of
about 40 weight %, in which methyl formate is represented in
Component B with a share of about 60 weight %.
14. Electrolyte solution according to the claims of 1 to 8, in
which Component A is represented by Ethylene carbonate with a share
of approx. 40 weight %, in which Component B is represented by
acetone with a share of approx. 60 weight %.
15. Electrochemical double-layer capacitor with electrodes and a
porous separator between it with the characteristic, that it
includes an electrolyte solution according to one of the preceding
claims.
Description
[0001] Electrochemical cells, such as double-layer capacitors, are
used in the range of capacitors, as they can implement concurrently
high capacitances at very small ESR. For example, when used as
temporary energy storage, double-layer capacitors have to release
or accept high energy connected to them in relatively short periods
of a few seconds with a flow that is not that high. So that this
can take place with as little loss as possible, the electrical
internal resistance of the capacitors has to be minimized.
[0002] The internal resistance of the double-layer capacitors,
along with the material of the electrode layers, the separator and
the cell structure, is dependent essentially on the conductivity of
the operating electrolyte. Electrolytes with a conductivity of more
than 20 mS/cm at room temperature are required for double-layer
capacitors of larger power density with which capacitors can work
with sufficiently low internal resistance.
[0003] Solutions of primary salts in organic solvents cover known
electrolytes for double-layer capacitors with cell tensions of more
than 2V. The primary salts are organic compounds or show organic
cations or anions, for example, on the basis of onium acids salts
with nitrogen, sulphur or phosphorous as the central atom. Even
other heterocyclical compounds with quaternary nitrogen atoms are
suitable as cations. Suitable anions are, for example, the complex
halide of boron or phosphorous, tetrafluoroborate, or
hexafluorophosphate. A high degree of disassociation of salts is
indispensable for the conductivity of these electrolyte solutions,
which is supported by a high polar solvent. Primary salt solutions
in pure solvents, like acetonitriles with high polar and low
viscose properties, are known electrolyte solutions for
double-layer capacitors, which achieve a conductivity of more than
20 mS/cm at 25.degree. C. In WO 99/60587, an electrolyte solution
with a conductivity of 36 mS/cm is revealed, which contains an
N,N-dialkyl-1,4-diazabicyclo[2.2.2]octane diamine salt as primary
salt and acetonitrile as the sole solvent.
[0004] The disadvantage of this highly conductive electrolyte
solution containing acetonitrile is the fact that it is easily
flammable and develops toxic hydrogen cyanide (HCN) in case of
fire. Capacitors with such electrolyte solutions present a
considerable risk in case of fire and moreover cause problems
during the clean up.
[0005] The function of the instant invention is to provide an
electrolyte solution with high conductivity, which avoids the
aforementioned disadvantages of known electrolyte solutions.
[0006] An electrolyte solution that attempts to solve this
characteristic is described in claim 1. Advantageous designs of the
invention can be seen from further claims.
[0007] An electrolyte solution in accordance with the invention
features a solvent compound, which does not develop any HCN in case
of fire and is comprised of components which are assigned to three
categories A, B and C. The most important element of the solvent is
the component A, which at least incorporates a solvent with higher
polarity. Solvents with high polarity mean here solvents, which
favorably show a dielectric constant (DK)>10. The dielectric
constant of a solvent can be determined in a decametre by methods,
known to an expert. They are, for example, shown in the
Rompp-Chemistry Lexicon (9.sup.th Edition) under the term
"Dielectric constant" (Pages 955-956), reference to which is made
here to the full text.
[0008] The inventors have now recognized the fact that the high
polarity alone of the solvent A in an electrolyte solution is not
enough to obtain a sufficiently high conductivity. In fact, a
number of high polar solvents possess a high viscosity, which is
often >1 cP, and this affects the ion movement of the primary
salts to be dissolved in it and prevents achieving a sufficiently
high conductivity of the electrolyte solution.
[0009] A further solvent of low viscosity is added under the
invention as a further element B, until combined with a sufficient
quantity of a primary salt, an electrolyte solution of sufficiently
lower viscosity is derived. The solvent of low viscosity seen as
component B shows an advantageous viscosity of <1 cP. The
viscosity of a solvent can be determined for example by means of an
Ubbelohde-viscosimeter.
[0010] It can be seen that maximum conductivity can be achieved at
a degree of thinning that is dependent on a solvent of component A,
or with a viscosity connected with it. This maximum conductivity is
not achieved with a solvent compound, which corresponds to the
maximum polarity expressed by the dielectric constant of the
solvent compounds of components A and B, but with a solvent mixture
which does not have maximal polarity but ideal viscosity or
thinning. The invention represents the best possible compromise
between the possible high polarities with the possible low
viscosity.
[0011] It will have an electrolyte solution, which shows a
determined conductivity at 25.degree. C with more than 20 mS/cm,
which does not release HCN if fire breaks out. Such conductivities
were possible till now only with solvent compounds with an
acetonitrile share of more than 20 weight percentage. The invention
thus shows for the first time a way to get electrolyte solutions
for double-layer capacitors, which are suitable as quick energy
temporary store, and which, in case of fire, do not develop any
HCN.
[0012] High polar solvents for component A could be selected from
Pyrrolidine, lactone, carbonates, sulfone, oxazolidinone,
imidayzolidinone, amide or nitrile. In an electrolyte solution
under the invention, it is better to contain component A in a
proportion of at least 30 weight percent. It is preferable if
component A, as a high polar solvent, is at least a cyclical
carbonate, which is easily available, cost-effective and has high
polarity. It is preferable if such a cyclical carbonate is at least
40 weight percentage of the entire electrolyte solution.
[0013] Starting with a suitable component A, the choice of
component B is far less critical, as it is dependent exclusively on
the compatibility with the components A and C and the reduction in
viscosity related with it. Prevalent low viscosity solvents can be
used as component B, as, for example, open-chained carbonates,
ketones, aldehydes, ester or substituted benzene; however, solvents
with sufficiently low vapour tension are preferable.
[0014] A further design of the invention could contain
acetonitrile, the content or portion of which in the entire
electrolytes is set at a maximum 20 weight percentage. With such a
low content of acetonitrile, the danger of hydrogen cyanide
developing if fire breaks out would be minimal.
[0015] Primary salts and alloys of primary salts can be selected as
component C from the group of quaternary ammonium borates, ammonium
fluoroalkylphosphate, ammonium fluoroalkylarsenate ammonium
trifluoromethylsulfonate, ammonium bis(fluoromethanesulfonyl)imide
or ammonium tris(fluoromethanesulfonyl)methide. In addition to
ammonium, other cations can be used as cations, which can be chosen
from the group of the pyridinium, morpholinium, lithium,
imidazolium, and pyrrolidinium. Apart from the above-mentioned
anions, perchlorate, tetrachloroaluminate or oxalatoborate, or
compounds of these anions, can also be used. For even higher
conductivities under the invention, melted salt with organic
cations, which are available at room temperature in a liquid state,
can be used. Such melted salts could be chosen on the basis of
imidazolium cations or pyrrolidinium cations. Because of the high
costs of these salts melted at room temperature, they are limited
to special applications only, where the cost factor does not play a
part. Good results with sufficiently high conductivities can also
be achieved with standard primary salts, for example with Tri or
Tetra ethyl ammonium tetrafluoroborate.
[0016] The invention is described in detail below using design
examples: the relevant Table 1 shows the compounds of 7 electrolyte
solutions under the invention together with the conductivity
determined at 25.degree. C. In all design examples, the same
primary salt tetraethylammonium tetrafluorobrate has been used in a
concentration of maximum 1.2 mol/l. Higher concentrations do not as
a rule increase the conductivity, but increase additional costs,
which could be avoided. The primary salt can also be substituted
with other primary salts without significant changes in
conductivity.
1 TABLE 1 Components A A/B B C [weight %] [weight %] [weight %]
[Mol/l] Example No: 1 2 3 4 5 6 7 Propylene- 40 24 40 40 carbonate
Ethylene- 40 37 25 20 40 40 40 carbonate Acetonitrile 20 26 26 20
20 .gamma.-Butyric- 20 lactone Diethyl- 37 carbonate Acetone 25 60
Methyl formate 60 Tetra ethyl- 0.9 1.0 0.9 0.9 1.2 0.9 0.9 ammonium
tet- ra-fluoroborate Conductivity at 23.9 25.0 33.1 24.1 27.9 31.0
33.4 25.degree. C. [mS/cm]
[0017] The solvent compounds are comprised of up to four different
individual solvents in the example, whereby some solvents of the
Group A as well as the Group B are to be imputed, and can be
applied to both categories. The apparent high proportion of
acetonitrile in examples 2 and 3 gets reduced in the total
electrolyte solvent, inclusive of the primary salt, to
approximately 20%, so that the danger of the development of HCN can
be classified as minor. The quantities of solvent components A and
B are in weight percentage, based on the composition of the solvent
specified. The quantity data for primary salt are based on
concentration, based on mol/l electrolyte solution. It shows that
all examples have conductivity values from here to 33.4 mS/cm,
which make them really suitable for double-layer capacitors to be
used in the service range.
[0018] Electrochemical double-layer capacitors are to be
impregnated with electrolyte solutions under the invention for
determining the electrochemical data. Its electrical data can be
determined and compared with that of known comparable electrolyte
solutions. The corresponding data is reproduced in Table 2:
2TABLE 2 HCN develop- Conductivity Salt Solvent ment (mS/cm) R
[.OMEGA.] C [F] (C.sub.2H.sub.5).sub.4NBF.sub.40.9 Aceto-nitril yes
54.2 9.8 139 mol/l 100% (C.sub.2H.sub.5).sub.4NBF.sub.40.9
.gamma.-Butyric- no 17.4 33.7 126 mol/l lactone 100% Example 2
Strongly 28.2 22.6 142 reduced
[0019] It shows that with the electrolyte solutions under the
invention, comparative conductivity can be achieved, as with the
known solutions, which contain a high concentration of
acetonitrile. Thus, comparatively lower resistances are achieved in
capacitors filled with it. As against the known electrolyte
solutions of high conductivity, the electrolyte solutions under the
invention resulted in no or starkly reduced development of hydrogen
cyanide.
[0020] To find a suitable electrolyte solution, the following
procedure is recommended. Take a primary salt--for example a
standard primary salt--and release it in a polar solvent of Group
A, until a given concentration to primary salt is achieved, for
example 0.5 mol/l. Thereafter, the polar solvent is thinned
continuously with a further lower viscose solvent of Group B,
whereby the primary salt concentration is kept constant. For all
compounds the conductivity is determined. It shows that an optimal
conductivity value can be reached at a certain thinning grade.
Thereafter, the content of primary salt is optimized, whereby
gradually its proportions are increased. This procedure shows that
at a certain optimal concentration value of the component C, no
further increase in conductivity can be achieved. For an
electrolyte under the invention, it is preferable to select the
lowest concentration in primary salt with optimal conductivity.
[0021] Principally it is naturally possible, for optimization of a
primary salt solution to go out in a lower viscose solvent
(component B and to add high polar solution component A) or to
increase the portion of the high polar solvent. As in the
electrolyte solution under the invention, the part of component A
usually is in the majority; the first recommended way is generally
the most advantageous, at least as the inspected primary salts are
not soluble in pure solvents of category B.
[0022] The procedure can be modified to that extent that as
component A can be a compound of various high polar solvents. To
thin the component A, and compounds of various lower viscose
solvents component B can be added.
[0023] In further examples, besides the above-named solvents
propylene and ethylene carbonate, .gamma.-butyrolactone and
acetonitriles and, 3 Methyl-2-Oxazolidinone can be used for
Component A. Besides the aforementioned solvents, Component B with
lower viscosity can be diethyl carbonate, acetone, methyl formate,
ethyl acetate and/or ethylmethylketone. Besides tetraethylammonium
tetrafluoroborate (C.sub.2H.sub.5).sub.4NBF.sub.4, the primary salt
can also be lithium hexafluorophosphate LiPF.sub.6.
3 TABLE 3 Component A A/B B C [weight %] [weight %] [Weight %]
[mol/l] Bps No. 8 9 10 11 12 13 14 15 16 17 18 19 PC 50 25 EC 50 50
40 40 50 25 40 40 70 OX 50 50 .gamma.-B AC 50 60 50 30 40 MF 50 50
60 50 50 30 20 EA 30 EMK 50 TBF 0.9 0.9 0.9 0.6 0.9 0.9 0.9 0.9 0.9
1.0 LP 0.9 0.9 LF 26.5 27.2 26.0 31.0 33.4 24.3 24.8 28.6 31.6 29.7
33.0 20.1
[0024] The abbreviations used in Table 3 are: PC Propylene
carbonate; EC Ethylene carbonate; OX 3-Methyl-2-Oxazolidinone;
.gamma.-B .gamma.-Butyriclactone; AC Acetone; MF Methyl formate; EA
Ethyl acetate; EMK ethylmethylketones; TBF Tetraethylammonium
tetrafluoroborate; LP Lithium hexafluorophosphate; and LF the
conductivity of the electrolyte solutions in mS/cm at 25.degree.
C.
[0025] The high conductivity of the electrolyte solutions in
accordance with the invention is noticeable in a lower ESR-value of
double-layer capacitors, which can be operated with this
electrolyte solution. Table 4 compares the electrical data of
traditional capacitors with propylene carbonate as sole solvent
(Example 21) with capacitors, which can be operated with three of
the four above named electrolyte solution, (Examples 11, 12 and 19
from Table 3).
4TABLE 4 Capa- ESR [100 Example Salt Solvent city/F Hz/m.OMEGA.] 21
1 M (C.sub.2H.sub.5).sub.4NBF.sub.4 100% propylene 112 39 carbonate
11 0.9 M (C.sub.2H.sub.5).sub.4NBF.sub.4 See Table 3 No. 11 101 18
12 0.9 M (C.sub.2H.sub.5).sub.4NBF.sub.4 See Table 3 No. 12 123 13
19 0.9 M (C.sub.2H.sub.5).sub.4NBF.sub.4 See Table 3 No. 19 121
23
[0026] Based on this table, it is clear that capacitors with
electrolyte solutions under the invention at approximate similar
capacity would show considerably lower ESR-values than known
capacitors with high polar but also higher viscose solvents.
[0027] With the recommended procedure, further electrolyte
solutions under the invention can be found, the composition of
which can strongly deviate from the example.
[0028] In any case, it is surprising, that the said high
conductivity of more than 20 mS/cm can be achieved with the solvent
compounds under the invention, which are not applied to maximal
polarity.
* * * * *